The oil and gas industry routinely carries out deepwater operations, such as laying underwater pipelines and hoisting loads for constructing floating or fixed infrastructures at sea. Pipeline laying is done using laying vessels equipped to fabricate the pipeline on board the vessel itself, and to lay the pipeline as it is fabricated. This is currently done using what are known as S-lay and J-lay methods. S-laying substantially comprises fabricating the pipeline on a substantially horizontal assembly line on the multipurpose vessel, and launching the pipeline off a curved laying ramp, in the course of which, the pipeline assumes an S shape between the vessel and the seabed.
In the J-lay method, the final stage in fabrication of the pipeline is conducted in a substantially vertical J-lay tower, from which the pipeline is launched and assumes a J shape between the vessel and the seabed. J-laying is preferable when working in very deep water, by subjecting the pipeline to less stress; whereas S-laying is preferable when working in relatively shallow water. S-laying, in fact, has the drawback of subjecting the pipeline to severe tensile stress caused by the movement of the laying vessel, but, if this is not considered a problem, is preferable by enabling faster output.
Crane vessels are also known, which have a weather deck and are equipped with heavy-duty cranes for constructing floating or fixed infrastructures at sea.
The Applicant's Patent Application WO 2008/148464 A1 describes a multipurpose vessel equipped with a heavy-duty crane incorporating a J-lay tower.
This solution has the big advantages of the J-lay tower allowing unimpeded operation of the heavy-duty crane, but has the drawback of not allowing the heavy-duty crane to cater fully to the J-lay tower.
Patent Application US 2002/0159839 A1 describes a vessel comprising a J-lay tower reclinable into a substantially horizontal position; and two heavy-duty cranes. In this case, the two cranes occupy practically the whole of the vessel, so little space is left on the weather deck for storing the pipe assemblies, or for the heavy loads handled by the heavy-duty cranes. Moreover, the J-lay tower is positioned with its working face facing away from the heavy-duty cranes, thus making cooperation between the cranes and the tower even more difficult.
The Applicant's Patent Application WO 00/05525 describes a multipurpose vessel equipped with two heavy-duty cranes at the bow; a J-lay tower at the bow; and a pipe assembly prefabrication line on the weather deck. This embodiment also impedes cooperation between the heavy-duty cranes and the J-lay tower, as when fitting the pipeline with valve assemblies and other parts much larger than and, at any rate, different from the pipes and pipe assemblies. Such parts are known as special parts (bulky items), differ in shape and/or size from the pipe assemblies, and cannot be handled using conventional pipe assembly handling equipment; whereas a heavy-duty crane is ideally suited to transfer them to the J-lay tower.
Known state-of-the-art vessels clearly fail to effectively combine the two underwater-pipeline laying and heavy-duty hoisting functions. In fact, very often, one function impedes the other.